The present invention relates to a computerized tomography apparatus and a method for reconstructing a cross-sectional image of an object.
Conventionally, an X-ray computerized tomography (CT) apparatus and a method for reconstructing a cross-sectional image of the object are known wherein an X-ray tube is moved around an object to be photographed for projecting a fan-beam of X-rays at a plurality of positions, X-rays transmitted through the object are detected by an X-ray detector and detected X-rays in the form of projection data are processed to perform image reconstruction, thereby providing a tomogram of the object. Such an apparatus and method are disclosed in, for example, U.S. Pat. No. 4,135,247.
FIG. 1 is a block diagram showing an example of a conventional CT apparatus.
An X-ray tube 11 is moved around the object and projects a fan-beam of X-rays at a plurality of positions. An X-ray detector 12 detects X-ray transmitted through the object. Detected X-rays in the form of analog projection data are acquired by a data acquisition system 13 and are converted to digital data. The projection data converted in the digital form are supplied to the subsequent stages which constitute a pipeline processor. More specifically, the projection data acquired by the data acquisition system 13 are supplied to a central processing unit (CPU) 14. The CPU 14 transfers the projection data to a corrector 16 through a CPU/processor interface 15. The corrector 16 compensates projection data. Corrected projection data from the corrector 16 are stored in a corrector/convolver (C/C) memory 17. The projection data stored in the C/C memory 17 are read out and supplied to a convolver 18. The projection data are then convolved with a deblurring function. Convolved projection data are then supplied to an image reconstructor 19 in which image reconstruction is performed for each data to obtain a final picture. The reconstructed image data are accumulated in an image memory 20. The reconstructed image data accumulated in the image memory 20 is read out and displayed at a CRT display 21.
In the CT apparatus of the type described above, detection of X-ray data will be described with reference to FIG. 2. The X-ray tube 11 opposes the X-ray detector 12. The X-ray detector 12 comprises a plurality of detecting elements D linearly aligned with each other. Each detecting element D receives an X-ray in a channel (path) formed by connecting the X-ray tube 11 and this detecting element and provides an output signal representative of X-ray intensity integrated along the channel. The X-ray tube 11 and the X-ray detector 12 are rotated on a circle in the same direction at a proper timing in units of predetermined angles .DELTA..theta.. One image reconstruction operation corresponds to one revolution. Every time they are rotated at an angle of .DELTA..theta., the X-ray tube 11 projects a fan-beam of an X-ray which is then detected by the X-ray detector 12.
In this case, the smaller the angle .DELTA..theta. is, the more the projection data is obtained upon one revolution of the X-ray tube 11 and the X-ray detector 12. As a result, the quality of the reconstructed image is improved.
However, when the number of projection data is increased, the number of X-ray projections is increased, and image reconstruction processing time is increased.
However, when the angle .DELTA..theta. is increased, the number of projection data upon one revolution of the X-ray tube 11 and the X-ray detector 12 is decreased. Therefore, image reconstruction time is shortened.
However, when the number of projection data is thus decreased (e.g., 300), noise is mixed in the reconstructed image in the form of streak.